"Like so many matches this season we appeared half asleep in the opening minutes," Casillas said after Real came back from a goal down and a poor first half performance to secure a 4-2 home victory over struggling Celta Vigo.
"They scored then we managed to fight back and open up a good lead, but we relaxed when we thought it was all over and allowed them to score again."
"It isn't a good habit, but I suppose it will do as long as we keep winning."
Despite Casillas's concerns about his team's performance the weekend's results could hardly have gone better for Real.
While second half goals from Ronaldo, Zinedine Zidane and Luis Figo sparked Real's recovery at the Bernabeu, title rivals Valencia went down 2-1 at struggling Espanyol and Deportivo Coruna lost 3-2 at home to a resurgent Barcelona.
What's more, Valencia and Deportivo play each other at the Mestalla next Saturday, while Real face what should be a fairly straightforward away trip to mid-table Racing Santander.
Coach Carlos Queiroz admitted that it had been a perfect weekend for his team but, like Casillas, he was anxious to stress that the title race was far from over.
"The victory against Celta was our most important result in recent weeks," he said. "It was vital to gain another three points and we are very satisfied with how it turned out."
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"We have a good advantage at the moment, but nothing more than that. A defeat could still change everything."
Real's comfortable lead in the Primera Liga means they remain on course for a possible treble.
They are strong favourites to win the King's Cup when they meet Real Zaragoza in the final on March 17 and have the upper hand in their Champions League knockout tie against Bayern Munich following their 1-1 draw in Germany in the first leg.
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